1. Field of the Invention
The present invention relates to an improved process for manufacturing aldehydes through hydroformylation of olefins by reducing catalyst deactivation in the regeneration of rhodium catalysts.
2. Description of the Background
On a commercial scale, hydroformylation of olefins is performed with cobalt or with rhodium catalysts. Here, the use of rhodium catalysts is mostly advantageous, as greater selectivity and product yields can be achieved thereby. However, compared to cobalt, rhodium is more expensive; in the hydroformylation of olefins to the corresponding aldehydes with rhodium catalysts the catalyst is a cost factor that is not insignificant. In order to increase economic efficiency, the specific catalyst consumption must be reduced. This factor is understood to be the quantity of catalyst that must be provided to the process during long-term operation in order to guarantee a constant activity level.
The rhodium-catalyzed conversion of olefins to the corresponding aldehydes occurs mostly in the homogenous liquid phase. With the hydroformylation of propene a process has been established in the meantime, wherein the catalyst is present dissolved in a second liquid phase; however, the applicability of this process to longer-chain olefins is limited.
With hydroformylation in the homogenous phase, wherein catalyst, olefins, products, solvents, and the like are present in one phase, the problem which arises is the separation of the catalyst from the products after the reaction is complete. This can be done by distilling the unconverted educt and the products. The catalyst, mostly dissolved in high-boiling constituents in the bottom, is then returned to the reactor. Distillation can be performed either continuously or discontinuously.
In the case of separation by means of distillation, a degree of decomposition or deactivation of the catalyst is often determined. In particular, in the hydroformylation of longer-chain olefins, distillation of the products can only be conducted at increased temperatures and/or reduced pressures because of the boiling points of the products.
Several methods are known for reducing rhodium deactivation during regeneration of the reactor discharge in hydroformylation processes.
EP 0272608 B1 describes a process wherein a rhodium catalyst having triphenylphosphine oxide ligands is utilized for hydroformylation. In the regeneration of the discharge from the reaction, triphenylphosphine (nine-fold quantity relative to rhodium) is added to the discharge prior to its distillation. The distillation residue contains rhodium complexes with triphenylphosphine as ligands, as well as triphenylphosphine and triphenylphosphine oxide. In this mixture the free and complexed triphenylphosphine is oxidized to triphenylphosphine oxide. This catalyst solution is returned to the reactor. Oxygen or a peroxide is utilized to oxidize the triphenylphosphine. Further variants of this method are known and described in JP 63 222 139, JP 63 208 540, DE 3 338 340 and JP 63 218 640.
These processes have the following disadvantages: Triphenylphosphine is consumed constantly. The equivalent quantity of triphenylphosphine oxide is produced by oxidation. In order to limit its concentration in the reactor, a discharge flow system is required by which again rhodium is discharged. An oxidizing apparatus is also necessary. The oxidation process involves costs for the oxidizer unless it is conducted with air.
Other processes using other phosphorus ligands which stabilize the rhodium are described in the relevant literature, such as U.S. Pat. No. 4,400,547.
U.S. Pat. Nos. 5,731,472 and 5,767,321 and EP 0 149 894 describe processes for the hydroformylation of n-butenes. Rhodium catalysts containing phosphite ligands and stabilized by addition of amines are disclosed in these patents. The drawback to the addition of amines is that the amines can act as catalysts for aldol condensation and thus the formation of high boilers is favored.
Hydroformylation of a C8 olefin mixture, manufactured by dimerizing butenes, under the catalysis of rhodium complexes and their stabilization with substituted phenols is described in JP-04-164042. The rhodium compound, ligand and stabilizer are used here in a molar ratio of 1/10/50. The disadvantages of this process are the costs for the stabilizer and the expense of separating it. A need, therefore, continues to exist for a process for the hydroformylation of olefins in which deactivation of the rhodium catalyst is extensively suppressed.
Accordingly, one object of the present invention is to provide a hydroformylation process in which deactivation of the rhodium catalyst is suppressed.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for manufacturing aldehydes, which comprises:
hydroformylating an olefin having 3 to 21 carbon atoms under an atmosphere of CO/H2 in the presence of a rhodium catalyst in a hydroformylation reactor;
upon discharging the reaction product from the reactor,
a) separating the discharged material into a gaseous phase and a liquid phase,
b) separating the liquid phase into a top fraction containing unconverted olefins and aldehydes and a bottoms fraction containing the rhodium catalyst, and
c) cooling the bottoms fraction below the temperature of the material discharged from the hydroformylation reactor and feeding a gas containing carbon monoxide into the bottoms fraction.